In such an apparatus, a piston is mounted so that it can move in a cylinder where it can be propelled, by the explosion of a mixture of air and a fuel gas injected, from a cartridge, into a combustion chamber and to drive an element, for example a fastener (a nail or an insert) or some other anchor. Apart from the bearing-against-something safety feature, that prevents any firing when the apparatus is not pressed against a receiving material, this kind of apparatus has a member, known as a cage, which, when the apparatus is pressed against something, and by way of a feeler that may be a moving insert guide, pushes a chamber sleeve tube back until it comes into abutment against the cylinder head that carries the spark plug intended to cause the explosion, so as to close the combustion chamber thus formed by this sleeve tube, the cylinder head and the crown of the piston. The cage and the sleeve tube may be connected to one another by screwing. Furthermore, the retreat of the cage is performed against the action of a return spring, to return the apparatus to the open position, also bearing against the piston cylinder.
When the apparatus is in the closed position, the combustion chamber is sealed, at the rear, by an O-ring seal between the cylinder head and the chamber sleeve tube and, at the front, by another O-ring seal between the piston cylinder and the chamber sleeve tube.
In the open position, after firing, the apparatus is vented and cooled by drawing fresh air in from the rear and, when there is a fan in the chamber, by allowing this fan to rotate, in order to accelerate the venting.
To avoid recirculating burnt gases into the chamber during venting it is possible to mount, on the rear part of the chamber sleeve tube, a flexible annular diaphragm that isolates the annular space between the chamber sleeve tube and the casing of the apparatus from the combustion chamber.
Now, known gas-powered apparatuses, in the open and venting position, have a needless and disruptive annular space that is detrimental to the actual efficiency of the venting. This space on the whole lies between the casing, the rear part of the chamber sleeve tube and an annular peripheral portion of the cylinder head.